There is a focus detecting device. An AF sensor stores an object image passing through an image taking lens as an object image signal. The focus detecting device detects a defocus amount based on the object image signal output from the AF sensor.
When a moving object is continuously focused, there is a function that detects the focus every single frame, which is single shooting of continuous shooting (rapid-fire shooting), predicts an object moving position in future from the viewpoint of the detection, and moves the image taking lens.
Since the lens is driven by taking into account the prediction calculation result by the object-position prediction function, even the moving object can be continuously focused.
For example, Japanese Patent Laid-Open No. 2001-021794 discloses the control that drives the image taking lens in association with the movement of the moving object.
The control is performed such that a focus detection operation is completed while the object image reaches the AF sensor by moving a mirror from an up state to a down state, and that the image taking lens is driven and an aperture stop is adjusted before an image pickup element is exposed to light by moving the mirror from the down state to the up state. The focus detection operation includes storing the object signal in the AF sensor, reading out the object image signal, performing correlative calculation based on the read out object image signal, and performing prediction calculation. Also, a time while the mirror is moved down is called AF available time.
To maintain the number of shooting times per unit time of continuous shooting (hereinafter, also referred to as frame rate), the focus detection operation has to be ended within a predetermined time. However, the storage of the object signal in the AF sensor may take a time depending on the shooting environment (object brightness, contrast), and the readout of the signal and the prediction calculation may take a time depending on the number of AF frames. As the result, the frame rate may become unstable. In particular, if rapid-fire shooting is performed at high speed, the AF available time that can maintain the frame rate is reduced, and hence the frame rate likely becomes unstable.
To maintain the frame rate, for example, PTL 1 discloses means for reducing the time for the focus detection by recognizing a time for storing electric charges in an AF sensor in a selected focus detection area and setting the upper limit for a time for storing electric charges in another AF sensor.
There is control that individually compares a storage signal stored in the AF sensor by photoelectric conversion with a predetermined signal level, and stops the storage operation if the storage signal reaches the predetermined signal level (hereinafter, referred to as storage monitoring control).
In the storage monitoring control of the AF sensor, a constant time is waited from when the storage of electric charges in each line sensor is started until the electric charges are read out, then the storage monitoring control of each of a plurality of AF sensors is ended, and then electric-charge readout control is performed.
FIG. 18A is an illustration showing the storage monitoring control when a storage signal of a line sensor A is read out before the storage of electric charges in a line sensor B is completed. Regarding the AF sensor, the storage monitoring control for the line sensors is stopped during readout of the line sensor A. Owing to this, even if the storage of electric charges in the line sensor B satisfies a storage completion condition, the storage of electric charges in the line sensor B cannot be stopped. Hence, the signal stored in the line sensor B may be saturated after readout of the line sensor A and before the storage monitoring control is started again. Therefore, as shown in FIG. 18B, the waiting time for the storage monitoring control is set to be a long time so that storage stop processing is properly performed even when the storage of electric charges in the line sensor B is completed at the timing of FIG. 18A. By setting the storage completion condition as described above, the line sensor B can be prevented from being saturated. Japanese Patent Laid-Open No. 8-152551 discloses the detail of the AF sensor.